Nonaqueous batteries, particularly lithium batteries, have an excellent reputation for power output relative to size. A lithium cell requires the use of a nonaqueous electrolyte in view of the high chemical reactivity of the lithium. Certain ethers have been found to be very good solvents for use in electrolytes for lithium cells. For example, 1,2-dimethoxyethane is currently used in many lithium cells. Ethers are, however, relatively flammable materials. Others have investigated the use of aliphatic esters such as methyl acetate and methyl formate but these have also been found to be highly flammable. The stability of these materials towards lithium is also questionable.
In order to improve a lithium cell, it would be desirable to have a material with a high flash point or reduced flammability and also a material with a high boiling point to improve the upper operating temperature limit of the cell.
In U.S. Pat. No. 4,804,596, assigned to Honeywell, methyl formate and methyl acetate have been used as electrolyte solvents. However, these materials are highly flammable and the cells have very low operating temperature limits because of the low boiling point of the solvent.
Diesters derived from two alcohols and a dicarboxylic acid have been reported as solvents or cosolvents for lithium batteries (Hitachi's published unexamined Japanese Patent Applications Sho 52-142761 through Sho 52-142765). Our tests with several of the diesters have shown very poor results. Most cells failed to discharge and the electrolyte salts tend to be insoluble. Only mixtures of certain dialkyl succinates with propylene carbonate discharged to any practical degree.